Hi there and very sorry to hear about your loss of poor Boo.
I feel that every home is different, every owner is different and every cat is different, so basically it's down to you.
Some owners grieve for a long time and take a long time to accept the idea of a new cat. I'm afraid I'm one of these - but I always feel that the greatest tribute a bereaved owner can pay the cat is to find another cat who needs that good home...contradictory or what? Lol.
My personal feelings are that a home should have at least three cats, finances and time permitting, so that one is never left alone - but I know of a neighbour who had two cats, siblings, and the girl was bullied horribly by the boy. After his death, on her own, she blossomed and came into her own, and was happy for the first time in her life...
And on the other hand girls can be the most horrible little bullies too, so a female may not be more un-threatening than a boy.
However, the cat is what matters most and from what you say I do feel that Sully would welcome company, and yes, in view of his youth, a kitten might be the best option, that he can 'parent' - as he will, and run around and play with. He's still very young and cats are communal animals...if you do decide to introduce another kitten there may be hissings and carryings on, but it doesn't take long for things to settle down.
Or they may surprise you. 30 years ago I knew a cat rescue worker who had a Siamese among her resident cats. Frequently she said that he was lonely because he had nobody of his own nationality to pal up with. I thought this was total sentimentality!
Eventually she got the offer of a homeless Siamese, an adult neutered male like her first boy. When she brought him home, Siamese No 1 was in the same room when she opened No 2's carrier. Si 1 promptly jumped into the carrier and virtually embraced his compatriot! They were inseparable after that!
They're all different and they'll always surprise you. I think, follow your gut instinct - oh and there are zillions of kittens out there at the moment - you will never replace Boo but do hope Sully finds a friend
. She was so affectionate and loving and our home feels so different without her. Sully seems to be coping okay with being on his own but has the occasional spell where he runs around crazily and seems to be looking for her. We are starting to think it might be a good idea to get another kitten as Sully is still only 9 months old and we worry he may soon get used to being the only cat around. 
